Essential Organic Chemistry
Paula Yurkanis Bruice
Chapter 4
Alkenes: Structure, Nomenclature,
Stability, and an Introduction to
Reactivity
Introduction
Alkenes contain a C=C double bond
H
H
H
H
C=C double bond consists of
sp2-sp2  bond
p-p  bond
Introduction
compared to alkanes, bond lengths decrease in alkenes
133 pm
H
116.6o
H
H
154 pm
108 pm
H

121.7
H
C C
C C
H
H
H
109 pm

109.6
H
H
compared to alkanes, bond angles increase in alkenes
Introduction
Typical representatives are
• Ethene, plant growth hormone, affects seed
germination, flower maturation, and fruit ripening.
H
H
H
H
Introduction
Typical representatives are
• citronellol, in rose and geranium oils
4
5
3
2
6 1
OH
7
8
7
8
6
5
4
3
1
2
Geranium “Mavis Simpson”
Introduction
Typical representatives are
• limonene, in lemon and orange oils
1
2
3
1
4
6
6
2
5
5
3
Citrus limon
4
Introduction
Typical representatives are
• -phellandrene, in oil of eucalyptus
1
2
3
1
6
2
6
5
3
5
4
4
Eucalyptus globulus
4.1 Molecular Formulas
Alkane: CnH2n+2
Alkene: CnH2n
• or CnH2n+2- 2P
• P = number of double bonds
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
+ 2H
Molecular Formulas
Alkane: CnH2n+2
Ring: CnH2n
• or CnH2n+2- 2R
• R = number of rings
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
+ 2H
Molecular Formulas
Alkene:
CnH2n+2- 2P-2R
P = number of double bonds
R = number of rings.
4.2 Nomenclature of Alkenes
The functional group is the center of
reactivity in a molecule.
The IUPAC system uses a suffix to denote
certain functional groups.
Nomenclature of Alkenes
1-1. Find the longest carbon chain.
1-2. Enumerate the carbons such that the
functional group, here the double bond, gets the
lowest possible number.
Nomenclature of Alkenes
2. Substituents are cited before the parent
longest chain, along with a number indicating its
position at the chain.
Nomenclature of Alkenes
3. If a chain has more than one double bond, we
first identify the chain by its alkane name,
replacing the “ne” ending with the appropritate
suffix: diene, triene, etc.
4. If a chain has more than one substitutent,
substituents are cited in alphabetical order.
Nomenclature of Alkenes
5. If the same number for alkene is obtained in
both directions, the correct name is the name
that contains the lowest substituent number.
Nomenclature of Alkenes
 6. A number is not needed to denote the position of the
double bond in a cyclic alkene because the double bond
is always placed between carbons 1 and 2.
 7. Numbers are needed if the ring has more than one
double bond.
Nomenclature of Alkenes
Remember that the name of a substituent is stated
before the name of the parent hydrocarbon, and
the functional group suffix is stated after that.
[substitutent] [parent hydrocarbon] [fucntional group suffix]
Nomenclature of Alkenes
4.3 The Structure of Alkenes
All six atoms of the double bond system are
in the same plane.
4.4 Cis-Trans Isomerism
Because rotation about a double bond does
not readily occur, an alkene such as 2butene can exist in two distinct forms.
cis/trans Isomers
sp2-sp2  bond
p-p  bond
side view
p-p  bond
front view
The p-p  bond restricts free rotation.
cis/trans Isomers
Upon rotation we lose p-p overlap, thus
rotation doesn’t happen (easily).
Consequently, geometrical isomers exist.
cis/trans Isomers
cis
trans
H
H
R
H
R
R
H
R
All substituents are
on one side
of  bond
All substituents are
on different sides
of  bond
cis/trans Isomers
3
5
4
2
1
cis-2-pentene
3
5
4
2
1
5
3
4
2
1
cis/trans Isomers
2
1
4
3
6
5
7
6
7
4
5
trans-3-heptene
2
3
2
1
6
7
4
5
3
1
4.5 The E,Z System of Nomenclature
For more than two substituents the cis/trans
system cannot be used.
A new system, the E/Z system is introduced.
To use the E/Z system we need to assign
priorities to each substituent on each
carbon.
E/Z System
high priority
high priority
low priority
high priority
low priority
low priority
high priority
low priority
In case high priorities
are on the same side,
we assign a
Z configuration.
In case high priorities
are on opposite sides,
we assign an
E configuration.
E/Z System- Rule 1
The relative priorities of the two groups depend on the
atomic numbers of the atoms bonded directly to the sp2
carbon. The greater the atomic number, the higher is the
priority of the group.
E/Z System
Priorities are first assigned based on atomic numbers.
1
I
H
2
I>C
F>H
2
H3C
F
1
E-configuration
1
I
F
1
I>C
F>H
2
H3C
H
Z-configuration
2
E/Z System- Rule 2
If the two substituents attached to the sp2 carbon start
with the same atom, you must move outward and
consider the atomic numbers that are attached to the
“tied” atoms.
E/Z System
If you can’t decide using the first atoms attached, go
out to the next atoms attached. If there are
nonequivalent paths, always follow the path with
atoms of higher atomic number.
1
H
H3C
CH2 OH
1
C O
H
2
H
CH2 CH3 2
path goes to O,
not H
H
comparison
stops here
C C
H
Z-configuration
path goes to C,
not H
E/Z System- Rule 3
If an atom is doubly (or triply) bonded to another atom,
the priority system treats it as if it were singly bonded
to two (or three) of those atoms.
E/Z System
path goes to C,
not H
Atoms in double bonds
are “replicated” at
either end of the double
bond.
C H
CH CH2
C C C
H H
H H
CH2 CH3
H
CH CH2 1
1 H3C
CH2 CH3 2
2
E-configuration
C C H
H H
4.6 The Relative Stabilities of Alkenes
Alkyl substituents that are bonded to the sp2 carbons
of an alkene have a stabilizing effect on the alkene.
The more alkyl substituents bonded to the sp2
carbons of an alkene, the greater is its stability.
Stability
The stability of alkenes depends
upon number of substituents
R
H
R
H
<
H
H
R
H
<
H
R
R
R
R
R
<
R
R
The more substituents, the more stable
Stability
Steric repulsion (Steric strain) is responsible for
energy differences among the disubstituted alkenes
H3C
H
H
H
H
H
>
>
H3C
H3C
H3C
CH3
H
CH3
4.7 How Alkenes React ; Curved Arrows
The functional group is the center of reactivity of a
molecule.
In essence, organic chemistry is about the interaction
between electron-rich atoms or molecules and electrondeficient atoms or molecules. It is these forces of
attraction that make chemical reactions happen.
A very simple rule:
Electron-rich atoms or molecules are attracted to
electron-deficient atoms or molecules!
Electrophiles vs Nucleophiles
Electrophile: electron-deficient atom or molecule that
can accept a pair of electrons.
H
CH3CH2
BF3
Nucleophile: electron-rich atom or molecule that has a
pair of electrons to share.
HO
Cl
CH3NH2
H2O
A very simple rule restated:
A Nucleophile reacts with an electrophile!
Electrophiles vs Nucleophiles
Nucleophiles:
Organic molecules with double bonds
(alkenes, alkynes) are also nucleophilic.
Examples:
CH3
H3C C
H3C
CH
Reactions
Alkenes are similar in structure and do
similar reactions.
• All contain a double bond
• All contain the same functional group
Reactions are categorized through different
types of mechanisms.
Reactions
Typical for unsaturated systems
is the addition reaction:
A+B  C
CH2 CH3
HBr(aq)
H2C C
CH3
C
CH3
C
H
CH2 CH3
H2C C Br
CH3
H
CH3
HBr(aq)
C
Br
C
H H
Reactions
A LOOK AT THE
REACTANTS
Reactions
CH2 CH3
+ HBr(aq)
H2C C
CH3
CH2 CH3
H2C
H
WHAT IS THE NATURE
OF THIS REAGENT?
Br
CH3
Reactions
electrophile
 
HBr + H2O
Br + H3O
Hydrogen bromide is a strong acid and forms
hydronium ions, H3O+, and bromide, Br–, when
dissolved in water.
H3O+ is positively charged,
thus it is electron deficient
it is electrophilic “electron loving”
Reactions
In the presence of an electron-rich species
the hydronium ion reacts:
H3C H2C
H
C CH2 + O H
H3C
H
H3C H2C
C CH2 + H2O
H3C
H
electrophile
A new positively charged species is formed.
Reactions
The newly formed species, a carbocation, is
again electron deficient, thus it is electrophilic.
CH2 CH3
H2C C
H
electrophile
CH3
Reactions
One species present that is rich in electrons is Br–.
Since Br– bears a negative charge it seeks for
neutralization.
It is nucleophilic (nuclei are positively charged).
Reactions
The two species, electrophile and nucleophile, combine
and form a new compound.
CH2 CH3
+ Br
H2C C
CH3
H
electrophile
nucleophile
CH2 CH3
H2C
H
Br
CH3
Mechanism
Summarizing our reaction, we realize
it is a 2-step mechanism
 
HBr + H2O
H3C
H
H2C
C
H3C
CH2 + O H
H
CH2 CH3
+ Br
H2C C
CH3
H
Br + H3O
H3C H2C
C CH2 + H2O
H3C
H
CH2 CH3
H2C
H
Br
CH3
STEP 1
STEP 2
Mechanism
Step 1 reaches a carbocation “intermediate.”
One new bond is formed.
Intermediates are species with a very short lifetime.
However, their stability (energy) often determines
the outcome of a reaction.
Step 2 completes the reaction by forming a second
bond. Again, it is the interplay between positively
charged (electrophilic) and negatively charged
(nucleophilic) species.
A Few Words about Curved Arrows
4.8 Using a Reaction Coordinate Diagram
(Energy Profile) to Describe a Reaction
Transition State
Transition state
The chemical species that exists
at the transition state, with old
bonds in the process of breaking
and new bonds in the process of
forming:
TS 1

TS 2
H3C
CH2

H2C C
H
CH3

Br
bond
forming
bond
forming
bond breaking H
O 
H
H
Reactions
Overall reaction coordinate